The use of phosphogypsum (i.e. gypsum obtained from the production of phosphoric acid from calcium phosphate and sulphuric acid) as a raw material for the purpose of manufacturing plaster has been the subject of numerous investigations. The installation of phosphoric acid factories in countries which do not have natural gypsum available, the progressive exhaustion of gypsum quarries and the fight against pollution of water courses has lent renewed interest to these investigations.
It has long been known that phosphogypsum cannot be used in its raw state and that it must first be subjected to a purification process for the purpose of removing those elements which interfere with its setting or adversely affect the properties of the plaster obtained. Such a purification process generally consists of a neutralisation of the free acid impregnating the gypsum and of treatment by hydrocyclone, flotation, autoclave baking or double calcination.